1. Field of the Invention
This invention relates to lead-acid batteries and, more particularly, to a method of assembling a bipolar lead-acid battery and the resulting battery.
2. Description of the Prior Art
Lead-acid batteries and cells have been known for a substantially long period of time and have been employed commercially in a relatively wide variety of applications. Such applications have ranged from starting, lighting and ignition for automobiles, trucks and other vehicles (often termed "SLI batteries") to marine and golf cart applications and to various stationary and motive power source applications (sometimes termed "industrial battery" applications).
The lead-acid electrochemical system has provided a reliable energy source which is capable of being manufactured in automated production while providing acceptable quality. However, one serious drawback of either the flooded or sealed, absorbed electrolyte, lead-acid batteries is the relatively low energy and power density (i.e., kilowatts/unit weight and watt-hours/unit weight, respectively) provided by such systems. It has long been a desire to provide an energy source with the reliability of a flooded or sealed lead-acid battery system while at the same time achieving much greater energy and power densities.
For this reason, considerable effort over at least the last 20 years has been directed to using lead-acid and other electrochemical systems in a bipolar design. In such a design, by definition, the positive and negative plates in some fashion share the same conductive grid or substrate.
U.S. Pat. No. 3,728,158 to Poe et alo discloses one type of a bipolar battery. Poe et al. thus state the venting of vertically stacked bipolar electrode cells presents problems not encountered in the venting of serially aligned cells such as in conventional lead-acid SLI batteries. In the conventional lead-acid batteries, the gasses generated in the cell rise to a space above the plates and from there are vented directly out of the top of each cell. Electrolyte entrainment in the gasses is minimal. In vertically stacked bipolar electrode batteries, it is stated that the gasses cannot simply be vented directly through the top of each cell. Poe et al. accordingly disclose a low silhouette, bipolar electrode battery stack in which the several cells in the stack are individually vented up the side of the battery to a venting manifold in a manner which precludes intercell electrolyte communication. Intercell dividers which prevent direct electrolyte communication between the positive plate and negative plate of any one half-cell element have a frame-like border preferably comprising a plastic material. The several half-cell elements are stacked with the contiguous frames appropriately sealed to one another by heat, solvents or adhesives.
U.S. Pat. No. 4,125,680 to Shropshire et al. discusses a variety of electrode structures that involve the use of plastic for some parts of the electrodes. U.S. Pat. No. 2,496,709 to Gelardin thus describes, as Shropshire et al. state, a duplex electrode formed of a metal plate with various types of carbon plastic coatings thereon, around which a plastic frame is injection molded. Stacks of these composite duplex electrodes are snapped together in a locking interengagement. U.S. Pat. No. 3,910,731 to Warszawski et al. and the patents cited therein are stated to describe molding a plastic frame around a preformed electrode. Other prior art referenced includes U.S. Pat. No. 2,416,576 to Franz et al., U.S. Pat. No. 2,966,538 to Bernot, U.S. Pat. No. 3,775,189 to Jaggard, and U.S. Pat. No. 3,941,615 to McDowall. Notwithstanding the prior art developments in electrochemical device design and fabrication, Shropshire et al. state that the need for easily fabricated, lightweight, multicell electrochemical devices and enhanced fabrication techniques still exists. The Shropshire et al. invention thus discloses a plurality of bipolar carbon-plastic electrode structures formed by first molding thin conductive carbon-plastic sheets from heated mixtures of specified carbon and plastic, and then establishing frames of dielectric plastic material around the sheets and sealing the frames to the sheets so as to render the resulting structures liquid impermeable. Various embodiments are illustrated. FIGS. 5 and 6 illustrate one embodiment of a stack of frames prior to and after being joined to one another. Each of the frames, except the end frame, contains a projection. FIG. 5 shows the stack prior to its elements being heat sealed to one another. Upon heat sealing, under pressure applied to the ends of the stack, the projections flatten out across the width of the frame surfaces and the resulting sealed electrochemical device is capable of retaining liquid as is shown in FIG. 6.
U.S. Pat. No. 4,964,878 to Morris discloses a method of making a recombinant lead-acid battery which comprises assembling stacks of plates in such a manner that a positive plate in a particular position in one stack is connected to a negative plate in the same relative position in an adjacent stack by a common substrate of the positive and negative plates. Highly porous, microfine glass fiber separators are positioned between the positive and negative plates and compressive forces are applied to the stack of plates by fixing a battery cover on a container.
In discussing bipolar lead-acid batteries, U.S. Pat. No. 5,068,160 to Clough et al. identifies various problems. One problem which Clough et al. address is the addition of electrolyte to the assembled battery. The thickness of bipolar cells is often significantly less than the thickness of conventional monopolar cells, and such reduced thickness makes filling each of the bipolar cells, which are to be isolated against fluid flow from one cell to the adjacent cell, with a controlled amount of electrolyte, quite difficult, particularly at fill rates used to satisfy commercial production schedules and/or using commercially available equipment. A second problem that Clough et al. address is the need to effectively limit the liquid and/or gaseous components from one bipolar cell from interfering with the functioning of another bipolar cell. It is noted that such cell-to-cell interference can result in a reduction in the overall efficiency of the bipolar battery, or even in battery failure. Yet another problem with bipolar batteries, Clough et al. state, is that of maintaining the spacing between positive and negative electrodes. It is noted that such spacing is particularly important in sealed maintenance-free oxygen recombinant batteries. However, as the dimensions of the bipolar plate surfaces associated with the positive and negative electrodes increase, the more difficult it becomes to maintain proper inter-electrode spacing. The '160 patent to Clough et al. discloses an assembly of plates, spacer members and frame elements preferably made of thermoplastic polymers which are bondable together. As shown in FIG. 4, an assembled battery is illustrated in which bipolar plates are bonded to a frame element. As shown in FIG. 2, the end plate includes a series of apertures. Each of the frame elements include six through-holes, each of which is isolated from the other adjacent through-holes of the individualized frame element. When the battery is assembled, these through-holes are aligned to form six conduits. When the apertures are opened, each of these conduits is in fluid communication with a different one of the open apertures and with only one cell.
Yet, despite the substantial advantages that could be achieved using bipolar batteries and cells and the substantial amount of work and attention directed to this type of battery over at least the last 20 years, it seems that bipolar lead-acid batteries have remained a very promising but elusive curiosity, largely remaining as a laboratory curiosity. Thus, there still exists the need for a well-designed bipolar battery which will achieve the enhanced energy and power densities that only a bipolar battery can provide while satisfactorily dealing with the diverse problems in assembly and design identified by the prior art. More particularly, a substantial need exists for a bipolar battery design composed of components that can be assembled in an automated, reliable fashion, while achieving a well-functioning battery in a cost-effective manner.
It is accordingly a principal object of the present invention to provide a method for assembling a bipolar lead-acid battery which is amenable to automated production at commercially acceptable production rates, and to the resulting bipolar battery itself.
Another object lies in the provision of modular bipolar battery components that are amenable to automated production techniques.
A further object of this invention provides a bipolar battery design that may be assembled in such a fashion as to minimize the handling of the components so as to minimize scrap loss and the like.
A still further object is to provide a bipolar lead-acid battery design and a method for assembling such bipolar batteries which exhibit the versatility required to accommodate widely varying voltage and capacity specifications necessary for the specific service applications.
Yet another object provides the bipolar battery which is reliable in design and provides satisfactorily leak-free, hermetic seals.
A still further object provides a bipolar lead-acid battery having a unique electrolyte fill/vent design capable of allowing commercially acceptable electrolyte fill rates.
Another and more specific object of the present invention lies in the provision of a bipolar, recombinant lead-acid battery of a design amenable to automated, commercial production.
These and other objects and advantages of the present invention will be apparent from the following description and drawings.